In a mobile communications system based on a base station, inter-base-station synchronization is closely related to synchronization quality for a wireless communications service. There are two types of existing inter-base-station synchronization technologies: One is to implement synchronization by using a built-in Global Positioning System (GPS), BeiDou Navigation Satellite System, or IEEE 1588v2, and the other is to implement synchronization based on assistance of a user terminal. Although the former type of synchronization technology can implement high-precision time synchronization, synchronization precision is dependent on accuracy of a counter frequency, and imposes relatively high requirements on performance of a target base station and a network. Therefore, it is difficult in implementation. The latter type of synchronization technology is implemented based on a user terminal that needs to be handed over between base stations. A manner is: When a user terminal initiates a request for a handover from a source base station to a target base station, the source base station and the target base station determine moments at which same random access information sent by the user terminal is received by the source base station and the target base station, so that a time difference between the source base station and the target base station is obtained, and further synchronization between the source base station and the target base station is implemented. However, there is not always a user terminal that meets such a requirement. If a synchronization period is short, there may be no user terminal that needs a handover in this period, and synchronization precision is affected. Another manner is: A user terminal sends data to both a source base station and a target base station, where sending time is added to the sent data, so that the source base station and the target base station obtain a time difference between the base stations based on the sending time and receiving time. However, in this manner, time information needs to be reported by a user, and more messages need to be reported by the user terminal. As a result, a UE-related protocol needs to be modified, and implementation difficulty is increased.